The coaxial cables commonly used today for transmission of RF signals, such as cable television signals and cellular telephone broadcast signals, for example, include a core containing an inner conductor, a metallic sheath surrounding the core and serving as an outer conductor, and in some instances a protective jacket which surrounds the metallic sheath. A dielectric surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. In many known coaxial cable constructions, an expanded foam dielectric surrounds the inner conductor and fills the space between the inner conductor and the surrounding metallic sheath.
One of the design criteria which must be considered in producing any coaxial cable is that the cable must have sufficient compressive strength to permit bending and to withstand the general abuse encountered during normal handling and installation. For example, installation of the coaxial cable may require passing the cable around one or more rollers as the cable is strung on utility poles. Any buckling, flattening or collapsing of the tubular metallic sheath which might occur during such installation has serious adverse consequences on the electrical characteristics of the cable, and may even render the cable unusable. Such buckling, flattening or collapsing also destroys the mechanical integrity of the cable and introduces the possibility of leakage or contamination.
Traditionally, the preferred material for the metallic sheaths used in coaxial cables has been aluminum. Aluminum has been selected because of its low cost and good mechanical and electrical properties. Nevertheless, despite its benefits, aluminum does have some disadvantages. In particular, aluminum is susceptible to corrosion at the connector interface which can cause intermodulation distortion of the RF signals. Furthermore, although aluminum is highly conductive, other metals exhibit greater conductivity than aluminum.
One alternative to aluminum as the outer conductor or sheath is copper. Copper possesses better electrical properties than aluminum. However, copper is more expensive and has a higher compressive yield strength than aluminum, which contributes to poor bending properties. For these reasons, copper has not been used traditionally as the sheath material for coaxial cables. The use of a thinner copper layer can reduce the cost, but thin copper sheaths are even more susceptible to buckling and are very difficult to process.